Detecting a counterfeit access point in a wireless local area network

ABSTRACT

In a wireless local area network, beacon frames are transmitted over the wireless local area network by one or more access points. The beacon frames are received at a detector in the wireless local area network. The received beacon frames are analyzed at the detector to detect a counterfeit access point in the wireless local area network.

This application is a continuation of U.S. patent application Ser. No.10/112,402, entitled “Automatic Call Distribution System Using ComputerNetwork-Based Communication,” filed on Mar. 29, 2002, incorporated byreference herein and assigned to the corporate assignee of the presentinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to wireless local area networks.More particularly, the present invention relates to detecting acounterfeit access point in a wireless local area network.

2. Description of the Related Art

Computers have traditionally communicated with each other through wiredlocal area networks (“LANs”). However, with the increased demand formobile computers such as laptops, personal digital assistants, and thelike, wireless local area networks (“WLANs”) have developed as a way forcomputers to communicate with each other through transmissions over awireless medium using radio signals, infrared signals, and the like.

In order to promote interoperability of WLANs with each other and withwired LANs, the IEEE 802.11 standard was developed as an internationalstandard for WLANs. Generally, the IEEE 802.11 standard was designed topresent users with the same interface as an IEEE 802 wired LAN, whileallowing data to be transported over a wireless medium.

Although WLANs provide users with increased mobility over wired LANs,the security of communications over a WLAN can vary for reasons that arenot present in wired LANs. For instance, a counterfeit access can poseas an authorized access point in the WLAN. Stations in the WLAN canmistakenly associate with the counterfeit access point and can sendconfidential information to the counterfeit access point, withoutknowing that the counterfeit access point is unsecure. Consequently, thecounterfeit access point can obtain confidential information fromstations in the WLAN. Accordingly, the presence of a counterfeit accesspoint can present security problems in a WLAN.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a counterfeit access pointin a wireless local area network is detected by receiving beacon framesat a detector in the wireless local area network, where the beaconframes are transmitted over the wireless local area network by one ormore access points. The received beacon frames are analyzed at thedetector to detect the counterfeit access point.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be best understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing figures, in which like parts may be referred to bylike numerals:

FIG. 1 shows an exemplary Open Systems Interconnection (OSI) seven layermodel;

FIG. 2 shows an exemplary extended service set in a wireless local areanetwork (“WLAN”);

FIG. 3 is an exemplary flow diagram illustrating various states ofstations in a WLAN;

FIG. 4 shows an exemplary embodiment of an access point sending a beaconframe;

FIG. 5 shows an exemplary embodiment of an access point and acounterfeit access point sending beacon frames;

FIG. 6 shows an exemplary flow diagram of a process for detecting acounterfeit access point in a WLAN; and

FIG. 7 shows another exemplary flow diagram of a process for detecting acounterfeit access point in a WLAN.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In order to provide a more thorough understanding of the presentinvention, the following description sets forth numerous specificdetails, such as specific configurations, parameters, examples, and thelike. It should be recognized, however, that such description is notintended as a limitation on the scope of the present invention, but isintended to provide a better description of the exemplary embodiments.

With reference to FIG. 1, an exemplary Open Systems Interconnection(OSI) seven layer model is shown, which represents an abstract model ofa networking system divided into layers according to their respectivefunctionalities. In particular, the seven layers include physical layer102 corresponding to layer 1, data link layer 104 corresponding to layer2, network layer 106 corresponding to layer 3, transport layer 108corresponding to layer 4, session layer 110 corresponding to layer 5,presentation layer 112 corresponding to layer 6, and application layer114 corresponding to layer 7. Each layer in the OSI model only interactsdirectly with the layer immediately above or below it, and differentcomputers 100 and 116 can communicate directly with each other only atthe physical layer 102. However, different computers 100 and 116 caneffectively communicate at the same layer using common protocols. Forexample, in one exemplary embodiment, computer 100 can communicate withcomputer 116 at application layer 114 by propagating a frame fromapplication layer 114 of computer 100 through each layer below it untilthe frame reaches physical layer 102. The frame can then be transmittedto physical layer 102 of computer 116 and propagated through each layerabove physical layer 102 until the frame reaches application layer 114of computer 116.

The IEEE 802.11 standard for wireless local area networks (“WLANs”)operates at the data link layer 104, which corresponds to layer 2 of theOSI seven layer model, as described above. Because IEEE 802.11 operatesat layer 2 of the OSI seven layer model, layers 3 and above can operateaccording to the same protocols used with IEEE 802 wired LANs.Furthermore, layers 3 and above can be unaware of the network actuallytransporting data at layers 2 and below. Accordingly, layers 3 and abovecan operate identically in the IEEE 802 wired LAN and the IEEE 802.11WLAN. Furthermore, users can be presented with the same interface,regardless of whether a wired LAN or WLAN is used.

With reference to FIG. 2, an exemplary extended service set 200, whichforms a WLAN according to the IEEE 802.11 standard, is depicted havingbasic service sets (“BSS”) 206, 208, and 210. Each BSS can include anaccess point (“AP”) 202 and stations 204. A station 204 is a componentthat can be used to connect to the WLAN, which can be mobile, portable,stationary, and the like, and can be referred to as the network adapteror network interface card. For instance, a station 204 can be a laptopcomputer, a personal digital assistant, and the like. In addition, astation 204 can support station services such as authentication,deauthentication, privacy, delivery of data, and the like.

Each station 204 can communicate directly with an AP 202 through an airlink, such as by sending a radio or infrared signal between WLANtransmitters and receivers. Each AP 202 can support station services, asdescribed above, and can additionally support distribution services,such as association, disassociation, distribution, integration, and thelike. Accordingly, an AP 202 can communicate with stations 204 withinits BSS 206, 208, and 210, and with other APs 202 through medium 212,called a distribution system, which forms the backbone of the WLAN. Thisdistribution system 212 can include both wireless and wired connections.

With reference to FIGS. 2 and 3, under the current IEEE 802.11 standard,each station 204 must be authenticated to and associated with an AP 202in order to become a part of a BSS 206, 208, or 210. Accordingly, withreference to FIG. 3, a station 204 begins in State 1 (300), wherestation 204 is unauthenticated to and unassociated with an AP 202. InState 1 (300), station 204 can only use a limited number of frame types,such as frame types that can allow station 204 to locate andauthenticate to an AP 202, and the like.

If station 204 successfully authenticates 306 to an AP 202, then station204 can be elevated to State 2 (302), where station 204 is authenticatedto and unassociated with the AP 202. In State 2 (302), station 204 canuse a limited number of frame types, such as frame types that can allowstation 204 to associate with an AP 202, and the like.

If station 204 then successfully associates or reassociates 308 with AP202, then station 204 can be elevated to State 3 (304), where station204 is authenticated to and associated with AP 202. In State 3 (304),station 204 can use any frame types to communicate with AP 202 and otherstations 204 in the WLAN. If station 204 receives a disassociationnotification 310, then station 204 can be transitioned to State 2.Furthermore, if station 204 then receives deauthentication notification312, then station 204 can be transitioned to State 1. Under the IEEE802.11 standard, a station 204 can be authenticated to different APs 202simultaneously, but can only be associated with one AP 202 at any time.

With reference again to FIG. 2, once a station 204 is authenticated toand associated with an AP 202, the station 204 can communicate withanother station 204 in the WLAN. In particular, a station 204 can send amessage having a source address, a basic service set identificationaddress (“BSSID”), and a destination address, to its associated AP 202.The AP 202 can then distribute the message to the station 204 specifiedas the destination address in the message. This destination address canspecify a station 204 in the same BSS 206, 208, or 210, or in anotherBSS 206, 208, or 210 that is linked to the AP 202 through distributionsystem 212.

Although FIG. 2 depicts an extended service set 200 having three BSSs206, 208, and 210, each of which include three stations 204, it shouldbe recognized that an extended service set 200 can include any number ofBSSs 206, 208, and 210, which can include any number of stations 204.

Under the current IEEE 802.11 standard, before a station 204 canassociate with an AP 202, station 204 first locates the AP 202. Withreference to FIG. 4, an exemplary system that can be used to locate anAP 202 using beacon frames in a WLAN is shown. More particularly,according to the current IEEE 802.11 standard, AP 202 can transmitbeacon frames 400 across transmission range 402. Stations 204 locatedwithin transmission range 402 can detect beacon frames 400. In addition,stations 204 can use information in beacon frames 400 to locate AP 202'sBSS 206, 208, or 210 (FIG. 2) at a later time.

Generally, beacon frames 400 can include information such as frame type,beacon frame interval/rate, sequence number, timestamp, capabilityinformation, SSID, supported rates, one or more PHY parameter sets,direct sequence (DS) parameter set, frequency hopping (FH) parameterset, and the like.

According to the current IEEE 802.11 standard, sending beacon frames 400from AP 202 can be optional. However, some functionality in the WLAN canbe lost if AP 202 does not send beacon frames 400. For instance, if AP202 does not send beacon frames 400, station 204 may not be able tolocate AP 202 by passively listening for signals from AP 202. Instead,station 204 can send a probe request to locate AP 202. However, morebandwidth and time can be required if each station 204 in the WLANindividually sends a probe request to locate AP 202. Furthermore, forroaming stations 204, if AP 202 does not send beacon frames 400periodically, the roaming stations 204 can send probe requestsperiodically in order to locate the AP. However, periodically sendingprobe requests from these roaming stations 204 can consume even morebandwidth and time. In addition, if AP 202 does not send beacon frames400 and station 204 does not send a probe request, then both station 204and AP 202 can be unaware of the other. Accordingly, although sendingbeacon frames 400 from AP 202 can be optional, sending beacon frames 400from AP 202 can improve the functionality of the WLAN.

However, sending beacon frames from APs in a WLAN can also compromisethe security of communications over the WLAN. As noted earlier, WLANscan provide users with increased mobility, in comparison to wired LANs,but the security of communications over a WLAN can vary for reasons thatare not present in wired LANs.

For instance, with reference to FIG. 5, a counterfeit AP 500 can obtainconfidential information from a station 204 by posing as an authorizedAP 202. More particularly, counterfeit AP 500 can transmit beacon frame504 across a transmission range 502. Beacon frame 504 can includeinformation such as frame type, beacon frame interval/rate, sequencenumber, timestamp, and the like. Stations 204 located within thistransmission range 502 can detect beacon frame 504. After detectingbeacon frame 504, station 204 can associate with counterfeit AP 500,without realizing that counterfeit AP 500 is not an authorized AP 202.Once associated with counterfeit AP 500, station 204 can transmitconfidential information to counterfeit AP 500.

In order to avoid detection as a counterfeit AP, a counterfeit AP 500can pose as an authorized AP 202. In particular, counterfeit AP 500 candetermine information about authorized AP 202, such as the SSID forauthorized AP 202, the MAC address for authorized AP 202, and the like.Counterfeit AP 500 can then be configured with the same SSID asauthorized AP 202. In some applications, counterfeit AP 500 can obtainand use the MAC address of authorized AP 202. In addition, counterfeitAP 500 can locate itself near authorized AP 202 to avoid detection inthe WLAN. In some applications, counterfeit AP 500 can transmit astronger signal across the WLAN in order to entice stations 204 toassociate with it instead of authorized AP 202.

Because counterfeit APs 500 can obtain confidential information fromstations 204 by posing as authorized APs 202, counterfeit APs 500 cancreate unacceptable security problems in a WLAN. Accordingly, detectingcounterfeit APs 500 in a WLAN can be used to improve security in theWLAN.

With reference to FIG. 6, an exemplary process for detecting acounterfeit AP is depicted. With reference to FIG. 5, assume for thesake of example that AP 202 is an authorized AP and that counterfeit AP500 is an unauthorized AP attempting to pose as authorized AP 202. Asdescribed above, AP 202 sends beacon frames 400 and counterfeit AP 500sends beacon frames 504 in an effort to associate with stations thatwould associate with authorized AP 202. As such, as also describedabove, beacon frames 504 can include similar information as beaconframes 400 in an effort to pose as beacon frames 400. For example,beacon frames 504 can have the same sender MAC address (i.e., the MACaddress of authentic AP 202) and the same beacon frame rate.

In step 600 (FIG. 6) of the present exemplary process, detector 506receives frames from APs having transmission ranges that includedetector 506. As such, in the exemplary scenario depicted in FIG. 5,detector 506 receives beacon frames 400 and 504 from authorized AP 202and unauthorized counterfeit AP 500, respectively.

In step 602 (FIG. 6), detector 506 measures the rate at which frames arereceived to determine a measured frame rate. For example, in oneconfiguration, detector 506 can count the number of beacon framesreceived during a period of time. For the sake of example, assume thatdetector 506 counts a total of 100 beacon frames, which in the exemplaryscenario depicted in FIG. 5 would include beacon frames 400 and 504,during a 5 second interval. As such, in this example, the measuredbeacon frame rate is 20 frames per second.

In step 604 (FIG. 6), detector 506 compares the measured frame rate tothe stated frame rate. As described above, the stated frame rate can beobtained from the information provided in the frame itself. In thepresent example, assume that the stated beacon frame rate in beaconframe 400 is 10 frames per second. As described above, the measuredframe rate is 20 frames per second.

In step 606 (FIG. 6), detector 506 determines if a counterfeit AP isdetected based on the comparison of the measured frame rate to thestated frame rate. Again, in the present example, the measured framerate is 20 frames per second and the stated frame rate is 10 frames persecond. As such, in the present example, detector 506 determines that acounterfeit AP has been detected based on the difference in the measuredframe rate and the stated frame rate.

With reference now to FIG. 7, another exemplary process for detecting acounterfeit AP is depicted. With reference to FIG. 5, assume again thatAP 202 is an authorized AP and that counterfeit AP 500 is anunauthorized AP attempting to pose as authorized AP 202. As alsodescribed above, unauthorized counterfeit AP 500 can obtain the MACaddress of authorized AP 202. Counterfeit AP 500 can then use the MACaddress of authorized AP 202 as the sender MAC address in beacon frames504 in an effort to associate with stations that would associate withauthorized AP 202.

In step 700 (FIG. 7) of the present exemplary process, detector 506receives frames from APs having transmission ranges that includedetector 506. As such, in the exemplary scenario depicted in FIG. 5,detector 506 receives beacon frames 400 and 504 from authorized AP 202and unauthorized counterfeit AP 500, respectively.

In step 702 (FIG. 7), detector 506 compares the sequence number of areceived frame to the sequence number of a previously received framewith the same sender MAC address. More specifically in the presentexample, when detector 506 receives a beacon frame, it determines thesender MAC address of the beacon frame. If the sender MAC address of thereceived beacon frame matches the sender MAC address of an authorizedAP, detector 506 compares the sequence number of the received beaconframe to the sequence number of a previously received beacon frame fromthe same authorized AP, which was stored earlier.

In step 704 (FIG. 7), detector 506 determines if a counterfeit AP isdetected based on the comparison of the sequence number of the receivedframe to the sequence number of a previously received frame. If thesequence number of the received frame is consistent with that of thepreviously received frame, then detector 506 saves the sequence numberof the received frame as the sequence number of a previously receivedframe. However, if the sequence number of the received frame is notconsistent with that of the previously received frame, then detector 506determines that a counterfeit AP has been detected.

More particularly, in accordance with current IEEE 802.11 standard, APssend frames with sequence numbers that follow an incremental pattern.For instance, assume that authorized AP 202 sends beacon frames 400having sequence numbers in ascending order such as 100, 101, 102, andthe like.

Assume that detector 506 first receives beacon frame 400 having sequencenumber 100. As described above, when detector 506 receives beacon frame400, it examines the sender MAC address of beacon frame 400 to confirmthat the sender MAC address matches that of an authorized AP, which inthis example is that of authorized AP 202.

Assume that beacon frame 400 having sequence number 100 is the firstbeacon frame received from AP 202. As such, because the sequence numberof the received beacon frame 400 can not be compared to that of apreviously received beacon frame 400, the sequence number of thereceived beacon frame 400 is stored as the new sequence number of apreviously received beacon frame 400.

Now assume that detector 506 receives a beacon frame 504 fromcounterfeit AP 500, which is unauthorized and attempting to pose asauthorized AP 202. Also assume that counterfeit AP 500 has sent beaconframe 504 using the sender MAC address of authorized AP 202. However,assume that the sequence number for beacon frame 504 sent by counterfeitAP 500 is 50. Accordingly, when detector 506 compares the sequencenumber of the received beacon frame, which in this example is 50, to thesequence number of the previously received beacon frame, which in thisexample is 100, they are not consistent. As such, detector 506determines that a counterfeit AP 500 has been detected.

If detector 506 determines that the sequence number of the receivedframe and the sequence number of the previously received frame areconsistent, then the sequence number of the received frame replaces thesequence number of the previously received frame, and the new sequencenumber is stored. For example, if the sequence number of the receivedframe is 101, then 506 stores 101 as the new sequence number of apreviously received frame.

With reference to FIG. 5, the exemplary processes described above fordetecting a counterfeit AP in a wireless local area network can beperformed using software and/or hardware installed on a detector in thewireless local area network. In one embodiment, the detector is astation in the wireless local area network. Additionally, the stationcan be mobile, portable, stationary, and the like. For instance, thestation can be a laptop computer, a personal digital assistant, and thelike. In addition, the station can be used by a user as a diagnostictool, by an administrator as an administrative tool, and the like, toassess the quality of communications in the WLAN.

One advantage of the present embodiment includes allowing the station topassively monitor the WLAN to detect a counterfeit AP. By passivelymonitoring the WLAN in this manner, the station can detect a counterfeitAP in the WLAN without burdening AP 202, consuming bandwidth, orinterfering with traffic over the WLAN.

Although the present invention has been described with respect tocertain embodiments, examples, and applications, it will be apparent tothose skilled in the art that various modifications and changes may bemade without departing from the invention.

1. A method of detecting a counterfeit access point in a wireless localarea network comprising: receiving beacon frames at a detector in thewireless local area network, wherein the beacon frames are transmittedover the wireless local area network by one or more access points; andanalyzing the received beacon frames at the detector to detect acounterfeit access point in the wireless local area network.
 2. Themethod of claim 1, wherein analyzing comprises: obtaining a statedbeacon frame rate from a received beacon frame; determining a measuredbeacon frame rate; and comparing the measured beacon frame rate and thestated beacon frame rate.
 3. The method of claim 2, wherein acounterfeit access point is detected if the measured beacon frame rateand the stated beacon frame rate are inconsistent.
 4. The method ofclaim 2, wherein obtaining a stated beacon frame rate from the receivedbeacon frame comprises: examining a beacon frame to obtain a beaconframe rate stated in the beacon frame.
 5. The method of claim 2, whereindetermining a measured beacon frame rate comprises: counting the numberof received beacon frames during a period of time.
 6. The method ofclaim 1, wherein analyzing comprises: obtaining a sequence number from areceived beacon frame; and comparing the obtained sequence number to asequence number of a previously received beacon frame.
 7. The method ofclaim 6, wherein a counterfeit access point is detected if the obtainedsequence number and the sequence number of the previously receivedbeacon frame are inconsistent.
 8. The method of claim 7, wherein acounterfeit access point is detected if the obtained sequence number andthe sequence number of the previously received beacon frame are notsequential.
 9. The method of claim 6 comprising: replacing the sequencenumber of the previously received beacon frame with the obtainedsequence number if the obtained sequence number and the sequence numberof the previously received beacon frame are consistent.
 10. The methodof claim 6, wherein the sequence number of the previously receivedbeacon frame is associated with a medium access control (MAC) address ofthe previously received beacon frame, and wherein analyzing comprises:obtaining a sender MAC address of the received beacon frame; andcomparing the sequence number of the received beacon frame to thesequence number of the previously beacon frame if the obtained MACaddress is the same as the MAC address associated with the sequencenumber of the previously received beacon frame.
 11. The method of claim1, wherein the beacon frames are received below a network layer in anOpen Systems Interconnection (OSI) model.
 12. The method of claim 1,wherein the beacon frames are sent and received according to the IEEE802.11 standard.
 13. The method of claim 1, wherein the detector is astation in the wireless local area network.